We propose to examine the relative biological effectiveness of heavy ion radiation on cell lines grown as multicellular spheroids. These spheroids have some properties associated with organized tissue systems. We will examine the mechanisms underlying the apparent fraction of radioresistant cells in spheroids. The role of cell cycle redistribution and anoxia will be evaluated and potential for repair of sub-lethal damage and potentially lethal damage will be determined. The equivalent of a "tumor cure dose" for this in vitro system will be measured, along with growth delay as a function of LET of the heavy ion radiation. Alteration of cell cycle distribution in regrowing tumors will also be evaluated.